1. Field of the Invention
The present invention relates generally to a device for performing a machining operation by means of a laser beam. More specifically, the invention relates to a laser beam machining device which can perform a precise machining operation, such as laser dulling operation for forming a predetermined pattern on uneveness with a desired roughness on a surface of a metal roll. Further particularly, the invention relates to a laser beam machining device which intermittently irradiates a laser beam onto a surface of a work object and discharges an assisting gas toward a point on the work to which the laser beam spot is irradiated.
2. Description of the Background Art
In the recent years, metal sheets, panels, plates which are beautifully painted, are widely used in various products. As a typical example of the painted metal sheet, the cold rolled thin steel sheet is usually produced by subjecting the cold rolled steel sheet to degreasing, annealing and temper rolling in order. In this case, the temper rolling is to be performed by means of work rolls having dulled surfaces. The work rolls having the dulled surfaces may form a proper or desired surface on the metal sheet surface for improving the galling resistance in the press forming operation.
When such metal plate, panel or sheet is used for a vehicular panel, particularly for a vehicular outer panel, the finish feeling after painting is a very important factor for evaluation of the vehicle per se since the external appearance of the vehicular body can directly appeal to the customer. There are various factors for determining the quality of the painted metal sheet, panel or plate. Among various factors, it is considered as particularly important to have a glossiness lessening irregular reflections on the painted surface and an image clarity defining few image strains. In general the combination of the glossiness and the image clarity is referred to as "distinctness of image".
It is known that the distinctness of image on the painted surface is determined depending upon the kind of paint and the painting process but is strongly influenced by the roughness of the surface of the material metal sheet, panel or plate. Hereafter, the word "metal sheet" is used for representing various forms of metal products, including metal strips, metal panels, metal plates and so forth. Namely, when the ratio of flat section occupied in the steel sheet surface is small and the uneveness section is relatively large, the ratio of flat section occupied in the painted surface becomes small and the uneveness becomes larger, and consequently the irregular reflection of light is caused to degrade the glossiness and the image clarity to lower image distinctness.
On the other hand, in order to form the metal sheet into a desired configuration, press forming may be performed for the metal sheet after the temper rolling operation. In this press forming, proper lubrication for reducing friction between press molding and the metal sheet is essential for making the press operation easy. In addition, the metal powder generated by friction between the molding and the metal sheet has to be trapped in order to avoid possibility of galling. In this view, the impressionformed on the metal sheet will effectively work for holding lubricant and for trapping metal powder.
Furthermore, in consideration of the smoothness of the painted surface, sufficient thickness of the paint layer is required for obtaining a satisfactory level of smoothness of the painted surface and image distinctness. If the metal sheet is too smooth, adherence of the paint onto the metal sheet surface is relativedly poor and limits the thickness of the paint layer which can be stably formed. In other words, by providing proper uneveness on the metal sheet surface, sufficiently high adherence of paint can be obtained to allow formation of a thicker paint layer. Therefore, in this view, it is preferable to provide a proper roughness on the metal sheet surface.
In general, the roughness of the metal sheet surface is represented by a center-line average roughness Ra. Further, it is well known that as the center-line average roughness Ra becomes larger, the magnitude of height difference between crest and concave portions becomes greater and hence the uneveness of the painted surface becomes greater to degrade image distinctness.
When the metal sheet is subjected to a temper rolling process with working roll dulled through the conventional shot blast process or discharge working process, it exhibits a rough surface composed of irregularly patterned uneven dulled sections, i.e. irregularly arranged crests and concave portions, as set forth above, wherein the flat section represents a very small proportion of the surface area. When painting is applied to such a metal sheet, the ratio of flat portion present in the painted area becomes small since the coating is generally formed along the surface configuration.
In order to improve over the problems in the prior art set forth above, there has been proposed a surface treatment process for the work roll by means of a laser beam. Such laser beam surface treatment processes for work roll have been disclosed in the Japanese Patent First (unexamined) Publication (Tokkai) No. Showa 56-160892, the Japanese Patent Second (examined) Publication (Tokko) No. Showa 58-25587, the Japanese Patent First Publication (Tokkai) No. Showa 54-61043, and the Japanese Patent First Publication (Tokkai) No. Showa 55-94790, for example. However, such prior proposed processes are not always successful to provide a satisfactory property for the work roll surface.